Implantable cardiac stimulation devices are commonly used to provide therapy to regulate the functioning of the heart. These types of devices include pacemakers and implantable cardioverter-defibrillators (ICD). These devices have become more versatile since their original development as these devices are better able to regulate the function of the heart to optimize the performance of the heart to meet the patient's current needs.
In particular, the more sophisticated pacemakers include sensors, which are capable of sensing the function of the heart and the activity level of the patient. This information is then used to adjust pacing parameters so that the heart is regulated to beat in the optimum manner for the particular patient given the sensed conditions. A typical method of determining the activity level of the patient is to position an accelerometer within the housing that contains the control unit for the implantable cardiac stimulation device. The housing is generally positioned within the body of the patient, e.g., under the pectoral muscle for a pectoral implant device, and the accelerometer provides a signal which is indicative of the level of activity of the patient.
The function of the heart is quite often monitored through one or more of the leads that are positioned within the chambers of the heart which provide an intracardiac electrogram (IEGM) signal to the controller that controls the operation of the implantable cardiac stimulation device. The IEGM therefore provides a signal to the controller, which is indicative of the frequency at which the heart is beating. The controller can then use the IEGM signal and the activity signal provided by the activity sensor to ascertain when and how electrical stimulation pulse therapy should be provided to the heart to optimize the function of the heart for the activity level of the patient. The controller can also use the IEGM signal to detect the occurrence of arrhythmias, which would necessitate the delivery of anti-tachycardia stimulation pulses, low or high amplitude shocks to convert the arrhythmia.
While the IEGM provides an indication as to the heart rate, the IEGM does not generally provide the controller with an indication as to the volume of blood being pumped by the heart. It is understood that the amount of blood being pumped by the heart is a factor not only of the heart rate, but also of the stroke volume of the heart. To optimize the hemodynamics of the heart, it is generally preferable that the largest volume of blood be pumped for a given heart rate. Hence, it is desirable that the controller receive some sort of signal as to the contractility or displacement of the heart which can then be used as a basis for determining the hemodynamic efficiency of the heart.
One such device is disclosed in U.S. Pat. No. 5,549,650 wherein an accelerometer is positioned within a lead that is located within one of the chambers of the heart. Preferably, this lead is attached to one of the walls of the heart so that movement of the wall of the heart is sensed by the accelerometer. The accelerometer then sends a signal to the controller, which is integrated twice. The integration of the lead accelerometer signal results in both a first signal indicative of the contractility of the heart and also a second signal, which is indicative of the physical displacement of the wall of the heart. This information can then be used by the controller to adjust the pacing parameters of the implanted device to improve the hemodynamic efficiency of the heart as this information is directly related to the volume of blood being pumped by the heart during each heartbeat.
While U.S. Pat. No. 5,549,650 discloses a device that is capable of providing information to a controller as to the volume of blood pumped during each heartbeat, it will be appreciated that mounting an accelerometer in a lead that is attached to one of the walls of the heart is both invasive and expensive. With many patients, the therapeutic benefit provided by the sensor disclosed in U.S. Pat. No. 5,549,650 would not be justified in light of the invasiveness and cost of mounting the sensor on the inner wall of the patient's heart. For these patients, the system disclosed in U.S. Pat. No. 5,549,650 does not provide a suitable solution for providing the controller with additional information to optimize the hemodynamic efficiency of the heart.
From the foregoing, it should be apparent that there is a need for a system that will provide information to the controller of an ICD or pacemaker relating to the hemodynamic performance of a patient's heart without requiring any additional implantation of hardware within the patient. In particular, there is a need for a device that will provide information about the contractility and displacement of the heart without requiring the implantation of an additional sensor within the heart of the patient.